During processing of silver halide color photographic elements, the developed silver is oxidized to a silver salt by a suitable bleaching agent. The oxidized silver is then removed from the element in a fixing step.
The most common bleaching solutions contain complexes of ferric ion and various organic ligands. One primary desire in this industry is to design bleaching compositions that are more compatible with the environment. Thus it is desirable to reduce or avoid the use of ferric complexes as bleaching agents.
Peracid bleaching solutions, such as those containing peroxide and persulfate bleaching agents, offer an alternative to the ferric complex bleaching solutions. They are less expensive and present lower chemical and biological demands on the environment since their by-products are less harmful.
While persulfate bleaching agents have low environmental impact, they have the disadvantage that their bleaching activity is slow and thus requires the presence of a bleaching accelerator. However, the most useful accelerators are thiols that have undesirable odors.
Because hydrogen peroxide reacts and decomposes to form water, a peroxide-based bleaching solution offers many environmental advantages over persulfate and ferric complex bleaching solutions. Many publications describe peroxide bleaching solutions, including U.S. Pat. No. 4,277,556 (Koboshi et al). The described peroxide bleaching agents are catalyzed by various metal ions and are generally stable in an acidic environment. However, these acidic bleaching solutions are not rehalogenating or silver retentive (that is, the developed silver can be oxidized to a soluble salt, and undeveloped silver halide remains in the element).
Because of the toxicity of silver, its level in effluent is highly regulated, and photofinishers are often required to recover silver from the fixing solution. Photofinishers would rather not have to recover silver from both a bleaching solution and a fixing solution because of the added expense. Thus, it is desirable that the bleaching solution be silver retentive by containing halide ion. Yet, raising the level of halide ion in the bleaching solution to make it silver retentive, deactivates the bleaching agent under these acidic conditions.
U.S. Pat. No. 4,301,236 (Idota et al) and U.S. Pat. No. 4,328,306 (Idota et al) describe acidic peroxide bleaching solutions that contain a ferric complex catalyst. The bleaching solutions are silver retentive, but the presence of the iron destabilizes the system because iron is a well-known catalyst for peroxide decomposition.
WO-A-92/07300 (published Apr. 30, 1992) and EP 0 428 101A1 (published May 22, 1991) generally describe alkaline peroxide bleaching solutions that are rehalogenating. Although these alkaline solutions are quite active toward silver oxidation, they suffer from poor stability and are considered useful only with high chloride (&gt;90 mole %) silver halide emulsions.
Despite all of the efforts of researchers in the art, no peroxide bleaching solution or method has been commercialized because of the various problems including vesiculation (blistering of the photographic element from the evolution of oxygen), poor bleaching efficiency and instability.
There remains a need, therefore, for highly efficient peroxide bleaching that does not suffer from the problems noted above. Particularly, it is desirable that such bleaching be carried out with stable solutions (that is, solutions that suffer little decomposition of the peroxide), and be highly efficient in bleaching a variety of photographic elements including those having emulsions having &lt;90 mole % silver chloride.